1. Field of the Invention
The invention relates to integrated circuits and more particularly to vertical insulated gate transistors.
The invention applies in particular, although not exclusively, to high-speed logic circuits and radio-frequency circuits. More generally, the invention finds an application in technologies below 0.07 micron.
2. Description of Related Art
The vertical transistor is a device that overcomes the limitations of the planar MOS transistor, in lengths less than 0.1 micron. Its conduction body consists of a silicon pillar insulated and covered by a gate. It therefore has at least two conduction interfaces. Accordingly, the current Ion and the transconductance per unit width are at least doubled. For sufficiently fine silicon pillars, with a dimension of the order of 50 nm, coupling between the gates is observed, reducing the effects of the short channels. This makes it possible to reduce the doping of the pillar, which is particularly favorable from the point of view of the current Ion. Also, conduction over a plurality of interfaces, in conjunction with the coupling of the gates, makes it possible to eliminate the need to form ultrafine gate oxides or high-permittivity dielectrics.
What is more, the vertical transistor is a technological platform particularly suitable for implementing a coating gate architecture with ultrashort dimensions. This is because the channel length in the vertical transistor is not fixed by the photolithographic resolution. It is therefore possible to form channels with very small dimensions using standard photolithographic equipment. Also, coating a projecting silicon pillar with a gate is much simpler than coating a thin silicon film buried in a substrate.
The person skilled in the art knows of many methods of fabricating a vertical insulated gate transistor, using different techniques to form the silicon pillar. In some cases, the silicon pillar is grown epitaxially in an open window in a dielectric stack. In other cases, the silicon pillar is etched anisotropically from the insulated substrate.
The latter approach draws its inspiration largely from steps of the conventional method of producing a planar transistor. In particular, forming the pillar by etching resembles etching the gate of a planar transistor. The pillar is doped after it is formed, although it could be doped during epitaxial growth or before etching. The source and drain regions are implanted in a self-aligned manner relative to the pillar. The source can also be implanted before epitaxial growth, in which case it is referred to as “continuous” (the source areas on either side of the pillar are joined together). The gate oxide is then formed on the flanks of the silicon pillar. The polysilicon gate is then deposited, doped and then etched.
This kind of approach, which is simple to implement, makes it possible to develop a CMOS line based on vertical transistors at reduced cost.
Although this approach is useful, it has serious shortcomings. One shortcoming is that the gate greatly overlaps the source and drain areas, with an oxide between them whose thickness is comparable to that of the gate oxide. This overlap represents a serious penalty, as the associated capacitors (that associated with the overlapping of the gate on the drain and that associated with the overlapping of the gate on the source) contribute to the total load capacitance of an individual cell. This represents a penalty in terms of the operating frequency of the logic circuits. Also, this state necessarily renders the vertical transistor inappropriate for radio-frequency applications necessitating high transition frequencies, since the transition frequency of an MOS transistor is directly proportional to the reciprocal of the overlap capacitances.
Thus the “anisotropic etching of the pillar” approach, which is simple and can be implemented at lower cost, is reduced in value because it does not take into account all the benefits of the coating gate architecture for high-speed logic applications and degrades radio-frequency performance.
One way to reduce the overlap capacitances is to decouple the growth of the oxide on the flanks of the pillar from the growth of the oxide on the substrate. However, it would be necessary to form an oxide on the substrate ten times thicker than the gate oxide for the overlap on the source to become negligible, and for all that the overlap on the drain would not be reduced. Also, existing techniques for reducing the overlap of the gate on the source or the drain still yield poor performance, especially in the case of a pillar formed by anisotropic etching.
Accordingly, a need exist to overcome the shortcomings.